定义：
激光器某些量随时间的演化行为，例如光功率和增益。

激光器的动力学行为时有腔内光场与增益介质之间的相互作用决定的。一般来讲，激光器功率会随着增益和谐振腔之间差值而变化，而增益的变化率时有受激辐射和自发辐射过程决定的（也可能是由淬灭效应和能量转移过程决定的）。 
采用一些特定近似，例如激光器增益不是太高，在连续光激光器中，腔内激光功率P和增益系数g之间的关系满足一下耦合微分方程： 

其中TR是腔内往返一周所需的时间，l为腔损耗，gss为小信号增益（给定泵浦强度情况下），τg是增益弛豫时间（通常接近于上能态寿命），Esat是增益介质的饱和吸收能量。 

在连续波激光器中，最关注的动力学过程是激光器的开关行为（通常包括输出功率尖峰形成）和工作过程中存在扰动时的工作状态（通常会产生弛豫振荡）。
在这些方面，不同类型的激光器具有非常不同的行为。例如，掺杂的绝缘体激光器极易产生尖峰和弛豫振荡，而激光二极管则不会。
在调Q激光器中，动力学行为非常重要，其中储存在增益介质中的能量会在发射脉冲时产生很大的变化。调Q光纤激光器通常具有很高的增益，存在一些其它的动力学现象。通常会引起脉冲在时间域上具有一些亚结构，这些结构不能由上面的方程解释。 
无源锁模激光器也可以采用类似的方程；这时第一个方程需要加入一个附加项，描述饱和吸收器的损耗。这种效应得到的结果是弛豫振荡的衰减减小。弛豫振荡过程甚至不衰减，这样稳态解变得不再稳定，激光器具有调Q锁模或者其它类型的Q开关不稳定性。

Definition: the temporal evolution of quantities such as the optical power and gain in a laser

The dynamic behavior of a laser is determined by the interaction of the intracavity light field with the gain medium. Essentially, the intracavity laser power can grow or decay exponentially according to the difference between gain and resonator losses, whereas the rate of change in the gain is determined by stimulated and spontaneous emission (and possibly by other effects such as quenching and energy transfer).

With certain approximations, including a not too high laser gain, the dynamics of the intracavity optical power P and the gain coefficient g in a continuous-wave laser can be described with the following coupled differential equations:

where T_R is the cavity round-trip time, l the cavity loss, g_ss the small-signal gain (for a given pump intensity), τ_g the gain relaxation time (often close to the upper-state lifetime), and E_sat the saturation energy of the gain medium.

Dynamic aspects of special interest in the case of a continuous-wave laser are the switch-on behavior (often including spiking of the output power) and its reaction to disturbances during operation (usually leading to relaxation oscillations). In these respects, different types of lasers exhibit very different behaviors. For example, doped-insulator lasers exhibit a strong tendency for spiking and relaxation oscillations, whereas for laser diodes this is not the case. Dynamic aspects are particularly important in Q-switched lasers, where the energy stored in the gain medium undergoes a large change during pulse emission. Q-switched fiber lasers, typically having a high laser gain, can exhibit additional dynamical phenomena. This can lead to a temporal sub-structure of the pulses, which cannot be explained with equations such as those given above.

Similar equations can be used for passively mode-locked lasers [1]; an additional term appears in the first equation, which describes the losses of the saturable absorber. The effect of this is typically that the damping of the relaxation oscillations is reduced. The relaxation oscillations may even become undamped, so that the steady-state solution becomes unstable, and the laser exhibits Q-switched mode locking or other kinds of Q-switching instabilities.

In optical amplifiers based on laser amplification, there can be a similar interplay of signal power and gain. In case of a regenerative amplifier, the similarity is strongest.

Bibliography